Automatically modify the BigQuery schema when appending data to an existing table

[arostamianfar]

We currently provide a "--append" option to append data to an existing table. However, it requires the schema to be exactly the same. We can use the BigQuery API to update the schema prior to import (in cases where the schema is compatible) or throw an error when the schema is incompatible (e.g. the same field is defined with a different type).